For optimizing the pollutant emissions and the exhaust gas aftertreatment, lambda sensors are used in modern internal combustion engines to determine the composition of the exhaust gas and to control the internal combustion engine. Lambda sensors determine the oxygen content of the exhaust gas, which is used to regulate the fuel-air mixture supplied to the internal combustion engine and thus to regulate the exhaust gas lambda upstream from the catalytic converter. Air and fuel feeds to the internal combustion engine are regulated via a lambda control loop, to achieve an optimal composition of the exhaust gas for the exhaust gas aftertreatment by catalytic converters provided in the exhaust gas duct of the internal combustion engine. Gasoline engines are usually regulated to a lambda value of 1, i.e., a stoichiometric ratio of air to fuel. Pollutant emission by the internal combustion engine may be minimized in this way.
Various forms of lambda sensors are in use. In the case of a two-point lambda sensor, also known as a discrete-level sensor or Nernst sensor, the voltage-lambda characteristic curve has an abrupt drop at λ=1. It therefore essentially allows a differentiation between rich exhaust gas (λ<1) during operation of the internal combustion engine with excess fuel and lean exhaust gas (λ>1) during operation with excess air and permits regulation of the exhaust gas at a lambda value of 1.
A broad-band lambda sensor, also referred to as a continuous or linear lambda sensor, permits measurement of the lambda value in the exhaust gas in a wide range around λ=1. Thus, an internal combustion engine, for example, may also be regulated to a lean operation with excess air.
Steady lambda regulation upstream from the catalytic converter is also possible by linearization of the sensor characteristic curve, even using a less expensive two-point lambda sensor in a limited lambda range. The prerequisite for this is that there must be a definite correlation between lambda and the sensor voltage of the two-point lambda sensor. This correlation must be present over the entire lifetime of the two-point lambda sensor since otherwise the regulation accuracy will be inadequate, and inadmissibly high emissions may occur. This prerequisite is not met, due to manufacturing tolerances and aging effects of the two-point lambda sensor. Two-point lambda sensors upstream from the catalytic converter are therefore mostly used together with a two-point regulation. This has the disadvantage that the target lambda is adjustable only with precontrol but cannot be regulated for catalytic converter testing, for example, or for protection of components in operating modes for which a lean or rich air-fuel mixture is necessary.
Various methods are believed to be understood for calibrating the voltage-lambda characteristic curve of two-point lambda sensors.
German patent document DE 38 27 978 discusses how a voltage offset of the existing voltage-lambda characteristic curve may be determined and compensated with respect to a reference voltage-lambda characteristic curve of the two-point lambda sensor, which is constant over the entire lambda range, by balancing the sensor voltage during overrun fuel cutoff of the internal combustion engine. Furthermore, DE 10 2010 027 984 A1 describes a method for operating an exhaust gas system of an internal combustion engine in which at least one parameter of the exhaust gas flowing in an exhaust gas duct is detected by an exhaust gas sensor. It is provided that, during an operating state of the internal combustion engine in which there is no fuel injection or combustion, fresh air is supplied to the exhaust gas duct upstream from the exhaust gas sensor via a fresh air supply assigned to the exhaust gas system, and the exhaust gas sensor is adjusted during and/or after the fresh air supply.
However, sufficiently good compensation of the voltage offset is possible by this method only when it is equally pronounced, not only during overrun fuel cutoff with appropriate oxygen-containing exhaust gas but also in the entire lambda range. This may be the case if the voltage offset is due to a single cause. In most cases, however, there are multiple superimposed causes for a deviation in the voltage-lambda characteristic curve with respect to a reference voltage-lambda characteristic curve. These may be differently pronounced in different lambda ranges, whereby the voltage offset changes as a function of the lambda value of the exhaust gas. In particular the causes may be pronounced to different extents in the lean and rich lambda ranges. Such a voltage offset, which is a function of lambda, cannot be compensated adequately by an adjustment during overrun fuel cutoff. Another disadvantage of this method is that modern engine concepts have fewer and fewer overrun phases, which limits the possibility of such an overrun adjustment.
The publication DE 38 37 984 discusses a method in which a shift in the lambda-1 point of the voltage-lambda characteristic curve may be compensated via a control function by using a second lambda sensor installed downstream. A deviation in the voltage-lambda characteristic curve may thus be corrected at least to λ=1.
The publication DE 198 60 463 discusses a method for ascertaining the composition of the fuel-air mixture of an internal combustion motor during operation at a predefined setpoint interval from λ=1, in which method the actual interval from λ=1 is ascertained by temporarily adjusting the composition and analyzing the resulting response of a lambda sensor. It is thus provided that                initially there is an abrupt adjustment by a defined value in the direction of λ=1, and next the lambda value is modified further at a defined rate of change until there is a response by the lambda sensor,        and that the actual interval is ascertained from the value of the abrupt adjustment, the rate of change and the time until the lambda sensor responds.        
This method permits determination of the actual lambda value which occurs in reality. If this actual lambda value differs from the lambda value expected on the basis of the measured output voltage of the lambda sensor, then an offset in the voltage-lambda characteristic curve may be inferred. The voltage-lambda characteristic curve may be corrected using the determined actual lambda value.
It is a disadvantage that dynamic effects in determining the actual interval of λ=1 are not taken into account. These dynamic effects may distort the result so much that it does not have the accuracy required for a steady lambda regulation using a two-point lambda sensor upstream from the catalytic converter, i.e., the accuracy required for detecting a characteristic curve offset.